Learning Objective

Identify and differentiate the receptor types involved in autonomic and somatic innervation (NN, NM, and M1–M3), and understand where each receptor subtype is located and functions within the nervous system.

The autonomic nervous system (ANS) and somatic motor system use distinct receptor types. The figure (conceptual) illustrates where major receptor types are found:

NN Receptors (Nicotinic Neuronal)

Location:

• Cell bodies in the autonomic ganglia of both:
  • Parasympathetic nervous system (PANS)
  • Sympathetic nervous system (SANS)
• Adrenal medulla (chromaffin cells)

Function:

• Fast excitatory transmission between the preganglionic and postganglionic neurons
• Stimulates catecholamine release (Epi, NE) from the adrenal medulla

NM Receptors (Nicotinic Muscle)

Location:

• Motor end plate of skeletal muscle
• Innervated by somatic motor neurons (not autonomic)

Function:

• Mediates depolarization of skeletal muscle → muscle contraction
• Target of neuromuscular blockers (e.g., succinylcholine, rocuronium)

Muscarinic Receptors (M1–M3)

Location:

• All organs innervated by postganglionic PANS nerves, including:
  • Heart (M2)
  • Smooth muscle (M3)
  • Glands (M3)
• Thermoregulatory sweat glands are innervated by sympathetic cholinergic fibers (M3)

Function:

• Smooth muscle contraction
• Glandular secretion
• Cardiac rate reduction
• Sweat production (via SANS cholinergic)